This is definitely the word to describe what Vivian does in the middle of the night when she is sleeping. I think she’s got a little Swiss Miss Cocoa action going on. It might even be a slight yoodel, I’ll get back on the verdict after a few more evenings.
chor·tle verb, -tled, -tling
–verb (used without object)
1. to chuckle gleefully.
–verb (used with object)
2. to express with a gleeful chuckle: to chortle one’s joy.
3. a gleeful chuckle.
American Heritage Dictionary
A snorting, joyful laugh or chuckle.
intr. & tr.v. chor·tled, chor·tling, chor·tles
To utter a chortle or express with a chortle.
[Origin: b. chuckle and snort; coined by Lewis Carroll in Through the Looking-Glass (1871)]